A tachograph, which for example, dependent on time, graphically records the history of the vehicle driving, such as the vehicle speed, driving time and stopovers, receives a digital signal from a speed sensor. The speed sensor is mostly located in the vehicle gearbox and scans the speed of the gearwheel on the output shaft of the gearbox. Typically, speed sensors with Hall detectors are used. The Hall detector is placed in close proximity of the gearwheel periphery at a distance from about 0.5 to 2 mm. During driving, the said gearwheel rotates in direct proportion to the vehicle velocity and always when the gearwheel tooth passes in front of the Hall sensor, the magnetic lines of force of the basic magnet of the Hall sensor are concentrated in the Hall detector center causing thus the increase in the Hall detector output voltage. On the contrary, when a gap between the gearwheel teeth passes in front of the Hall sensor, the magnetic lines of force are diluted and the intensity of the magnetic field decreases what results in the decline of the Hall detector output voltage. The output voltage signal from the Hall detector is analog and in the electronic circuit of the Hall sensor the analog signal is converted to a digital signal, which proceeds to tachograph, where the signal is processed to show a graphical or another record of the vehicle driving history. The conversion of the analog signal, which when scanning the rotating gearwheel, is substantially of sinusoidal character, to a digital signal, occurs only when the predetermined upper and lower threshold value of the output voltage from the Hall sensor is exceeded, which output signal is a function of intensity of the magnetic field in the center of the Hall sensor. When the basic magnetic field of the Hall sensor is affected by a disturbing magnetic field generated for example by attaching a magnet with a strong disturbing magnetic field to the gearbox case near the speed sensor, both the magnetic fields interfere with each other so that the intensity of the resulting magnetic field in the center of the Hall detector either increases or drops dependent on the mutual polarity of both magnets, i.e. that of the disturbing magnet and the magnet of the Hall sensor. The result is that the output voltage in the form of the analog signal from the Hall detector increases or drops so that the predetermined threshold values of the output voltage at which the analog signal is converted to a digital signal are no more exceeded and the output digital signal of the speed sensor has a constant value. Due to the absence of any change or frequency of the digital signal respectively, the tachograph records the stationery mode of the vehicle even when the vehicle is moving.
Known are also speed sensors with the Hall sensor and electronic processing of the analog signal to the digital signal, where the upper and the lower threshold values the excess of which causes the conversion of the analog signal to the digital signal, are self-adjusting with respect to the upper and lower peeks of the analog signal so that the conversion of the analog signal to the digital signal is missing only in the event when the intensity of the magnetic field in the center of the Hall detector is out of the sensitivity of the Hall sensor i.e. under ca 10 mT or above ca 500 mT in absolute values. On the contrary, when the intensity of the magnetic field in the center of the Hall detector is in its range of sensitivity, i.e. between ca 10 mT and ca 500 mT, the analog signal is always converted to the digital signal. In this situation, the strength or intensity, respectively, of the magnetic field in the center of the Hall detector is determined by the interference between the magnetic field of the magnet of the Hall sensor and the magnetic field of the disturbing magnet located in close proximity to the magnetic speed sensor using the Hall sensor—see for instance US 2011/0251805 A1.
US 2012/009000 A1 discloses a speed sensor with two detecting elements of different sensitivity producing two output signals, which, when the detecting elements are exposed to disturbing magnetic fields differ from each other so that the detecting element with a lower sensitivity issues undistorted signal corresponding to the actual speed and the detecting element with a high sensitivity gives a distorted signal, inadequate to the actual speed.
U.S. Pat. No. 6,271,663 B1 describes a rotational detector for detecting a rotational position comprising several magneto-resistive detectors arranged at angular intervals on a circle circumscribing the rotational axis of a rotor. The magnets of detectors are arranged so that their axes are disposed radially and not aligned with the sensitive axis of the magneto-resistive detectors whereby the polarities of adjacent magnets are opposite.
The common disadvantage of the above described speed sensors is that when they are exposed to an external disturbing magnetic field, the magnetic field in the center of the detector may be diminished or amplified to the extent that it comes outside the sensitivity range of the Hall detector and the output analog signal is missing. At the present time, the Hall sensors available on the market respond to the intensity of the magnetic field in the range of sensitivity from ca 10 mT to ca 500 mT.
The object of the present invention is to provide a speed sensor for measuring the speed of a gearwheel inside a gearbox, especially that of motor vehicles, which is not susceptible to external disturbing magnetic field of an available permanent magnet placed on an external surface of the gearbox in close proximity of the speed sensor and which provides exact and undistorted information about the speed of the gearwheel or a magnetic sign sensed by such a speed sensor.